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Stability of the thermohaline circulation in analytical and numerical models

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Citation:
Lohmann, G. (1996): Stability of the thermohaline circulation in analytical and numerical models , PhD thesis, University of Bremen.
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Abstract:

Freshening of high latitude surface water in the North Atlantic can change the poleward oceanic transport of heat and salt with drastic effects on the global climate. The sensitivity of the thermohaline circulation is analyzed with respect to these perturbations. The study is based on coupled ocean-atmosphere (-sea ice) models with different levels of complexity in idealized geometries of the Atlantic ocean.An atmospheric energy balance model (EBM) is constructed which can predict the heat and fresh water fluxes at the surface. The response of the EBM to sea surface temperature anomalies and radiative forcing is consistent with complex atmospheric models.For a range of coupled models it is shown that the atmospheric transports affect the stability of the thermohaline circulation (THC). Coupled atmosphere EBM-ocean circulation model experiments show that the atmospheric heat transport is an important destabilizing effect while changes in fresh water flux are of minor importance for the THC. To understand the sensitivity of the THC for a range of atmospheric boundary conditions, a box model is designed, as it is considered the most simple atmosphere-ocean system. The analytical investigation shows how the stability of the THC is affected by the representation of the atmospheric transport of heat and moisture and the basic state.Depending on the meridional gradient in salinity, self-sustained oscillations do appear in a coupled atmosphere EBM-ocean circulation model caused by strong horizontal salinity gradients. It was found the the oscillatory state is more sensitive to perturbations than basic states with moderate meridional salinity gradients which is consistent with the analytical model.The sensitivity and feedback mechanisms affecting the THC are examined in a coupled ocean-atmosphere-sea ice system. The EBM is coupled with an ocean circulation model which includes a thermodynamic sea ice model. Due to a perturbation in high latitude salinity, the THC evolves into an other steady state with decreased atmospheric temperature, more sea ice, enhanced atmospheric heat transport, and decreased oceanic heat transport. The formation of intermediate water and cessation of deep convection in the northern North Atlantic is consistent with paleoclimatic findings of well documented climate shifts caused by a fresh water release.

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